1. Field of the Invention
This invention relates to the field of urea purification, and in particular, it relates to methods for removing biuret from urea.
2. Description of the Art
Urea is a widely used fertilizer and chemical precursor. Most often it contains some biuret that forms during the urea manufacturing process or when urea is otherwise heated to 130.degree. C. or above. Biuret can interfere with chemical processing and is toxic to many plants. Its phytotoxicity has been thoroughly studied, and it is regulated and monitored by government agencies and industry. For instance, the Indian government prohibits the import of urea containing more than 2 weight percent biuret. The United States agricultural industry generally observes an upper limit of 0.25 weight percent biuret for urea fertilizers classified as "low biuret." This criterion is generally recognized by the citrus and other industries that use urea for foliar fertilization.
Detectable biuret toxicity symptoms have been noted in field tests on lemon and grapefruit in Southern California at biuret levels as low as 0.1 weight percent. Biuret toxicity has also been observed with topically applied urea prills and solutions. Seed germination inhibition and damage to seedlings has been observed in wheat, barley and similar grain crops at levels of 2 weight percent biuret.
Damage to corn has been observed at foliar biuret dosages of 0.2 to 0.5 kilogram per hectare. Thirty percent yield loss was noted in one study at 1.7 kilograms biuret per hectare banded near seeds. Wheat damage has been observed at 0.2 to 0.5 kilogram per hectare foliarly applied, and severe toxicity was observed at 6.0 kilograms per hectare biuret banded in the soil. Fifteen to twenty ppm soil biuret level has been shown to inhibit barley seed germination while substantial crop damage from foliar application often occurs at 0.4 to 0.6 kilogram biuret per hectare.
Similar effects have been observed in rice, citrus, cotton, avocado, beans, soybeans and potatoes, several of which are particularly sensitive to biuret in foliar fertilizers. In citrus, as little as 0.2 kilogram foliarly applied biuret per hectare causes detectable damage. Avocados are damaged by as little as 50 ppm biuret in foliar sprays. As little as 3 kilograms per hectare biuret banded in the soil inhibits potato germination and causes citrus damage in light soils. These studies, and a comprehensive review of the literature available on this subject, are presented by Mithyantha, Kulkarni, Tripathi and Agnihothrudu, Fertilizer News, 1977, pp. 13-18.
In view of these results, it is not surprising that the industry has devoted substantial effort to methods of preventing biuret formation in the first instance, and to methods of reducing its concentration once it is formed. Most contemporary commercial urea plants are capable of producing solid and solution urea containing much less biuret than was previously the case. However, essentially all commercial ureas contain at least 0.5 weight percent biuret, and most contain from 1 to 2 weight percent biuret. Biuret content can rise considerably higher if manufacturing conditions are not adequately controlled.
One method for removing biuret from urea solutions is described by Fuentes et al. in U.S. Pat. No. 3,903,158, issued Sept. 2, 1975. Fuentes et al. describe a procedure in which a urea solution containing biuret is passed over either anionic or cationic exchange resins which, according to Fuentes et al., selectively retain biuret and allow urea to pass through the resin. The exchange resin can then be regenerated by contact with a basic solution after which the resin can be reused.
Another process for removing biuret from urea, disclosed by Young and Green in U.S. Pat. No. 4,345,099, involves treating a biuret-containing urea solution at a pH of about 12.5 or higher and a temperature of about 0.degree. C. to about 100.degree. C. under which conditions the biuret is hydrolyzed and thereby eliminated from the solution.
Takahashi and Yoshida, in "Determination of Biuret in Urea by Ion Exchange Resins", Soil and Plant Food, Volume 3, No. 3 January 1958, pages 142-144, disclose a process similar to that described by Fuentes et al., supra, in which biuret is removed from aqueous urea solutions by contact with a basic anion exchange resin. According to Takahashi et al., the biuret is quantitatively retained on the resin, even after water washing, thus allowing quantitative determination of biuret in aqueous urea solutions. The resin can be regenerated by acidic solutions, such as hydrochloric acid solutions, which contain chloride ion.
Another procedure for removing biuret from urea which is sufficiently quantitative to allow for its use as an analytical procedure, is disclosed by Geurts, Steele and Brinkman in "Determination of Biuret in Urea Mixed Fertilizers," Analytica Chimica Acta, Volume 41, (1968) at pages 113 through 120. Geurts et al. disclose that biuret, which is first complexed with copper while in solution with urea, can be quantitatively removed from the solution by contact with certain ion exchange resins, and that the copper-biuret complex is not displaced from the resin by 0.9 molar ammonia or 0.3 molar sodium hydroxide solutions but can be eluted with 2 molar potassium nitrate followed by 0.2 molar nitric acid extraction.
General references to the characteristics and utility of anion exchangers such as strongly basic anion exchange resins are found in the trade literature such as Rohm & Haas Product Bulletin "Amberlite" IRA-400, Bulletin IE-16-56, revised April 1956, which discloses that Amberlite IRA-400 is a strongly basic anion exchange resin which can extract negative ions from either acidic, neutral or basic solutions. Biuret is known to be negatively charged in aqueous solutions. Thus, the Rohm & Haas bulletin suggests that strongly basic anion exchange resins such as Amberlite IRA-400 are capable of removing negatively charged ions such as biuret from either acidic, neutral or basic solutions.
Against this background, it can be seen that biuret can be effectively removed from aqueous urea solutions by the use of basic anion exchange resins, and that such methods require the use of relatively expensive anion exchanger and regeneration techniques.
Strongly basic anion exchangers such as Amberlite IRA-400 cost in the range of about $50 to about $150 per cubic foot. The strongly caustic or acidic solutions required to regenerate the exchangers are also relatively expensive. These regenerants must be sufficiently strong to dislodge the biuret from anion exchanger. Since, according to the literature, the biuret is relatively strongly held by the anion exchanger (a feature which would be beneficial from the standpoint of assuring adequate removal of biuret from the urea solution), the art suggests that relatively severe regeneration conditions are required to efficiently remove the biuret from the deactivated anion exchanger. Obviously, the cost of anion exchanger regeneration, the cost of constructing, maintaining and operating a system capable of removing biuret from a certain quantity of urea solution, and the expense of the anion exchanger required in the process, all increase as the frequency and/or severity of regeneration increases. Thus, the requirement for frequent and/or more severe regeneration increases the regenerant costs and the amount of anion exchanger and the size of the operating facility required to treat a given amount of urea solution.
We have now found that the frequency and severity of regeneration, the quantity of required anion exchanger, and the size of the operating plant required to remove biuret from aqueous urea solutions can be significantly reduced by employing novel biuret exchange and exchanger regeneration procedures which increase the efficiency and rate with which biuret is removed from a biuret-containing urea feed solution and which reduce anion exchanger deactivation.
It is therefore one object of this invention to provide improved methods for removing biuret from aqueous urea solutions.
Another object is the provision of methods for removing biuret from urea solutions by ion exchange which reduce the rate and severity of anion exchanger deactivation.
Another object is the provision of methods for removing biuret from urea by ion exchange which increase the useful life of the anion exchanger.
Yet another object is the provision of methods for removing biuret from urea by ion exchange in which the frequency and duration of exchanger regeneration are reduced.
Yet another object of this invention is to reduce the cost of regenerant disposal associated with the removal of biuret from urea by ion exchange in which the ion exchanger is regenerated with basic regenerant.
Yet another object of this invention is to increase the rate of production of low biuret urea from biuret-containing solutions.
Yet another object is the provision of methods for the removal of biuret from urea by ion exchange which reduce the exposure of the ion exchanger to deactivating components.
Other objects, aspects and advantages of this invention will be apparent to one skilled in the art in view of the following disclosure, the drawing, and the appended claims.